Shallow Seamounts Research Articles

Eleven new heteroscleromorph Demospongiae (Porifera), and a new record of the tetractinellid Ancorina corticata, from Walters Shoal, a shallow seamount on the Madagascar Ridge in the South West Indian Ocean (SWIO)

Walters Shoal is a shallow seamount located on the South Madagascar Ridge, which is situated approximately 833 km south of Madagascar and 1,296 km east of South Africa. Even though it penetrates the euphotic zone (summit is 15 m below the sea surface) and is protected by the Southern Indian Ocean Deep-Sea Fishers Association, there is a paucity of biodiversity and oceanographic data. Thus, a multidisciplinary cruise was initiated in May 2014 on the FRS Algoa as a component of the African Coelacanth Ecosystem Programme (III). This paper reports on 11 new heteroscleromorph Demospongiae collected from Walters Shoal, namely Chelotropella alexrogersi sp. nov., Penares waltershoalensis sp. nov., Tethya muricyi sp. nov., Agelas kellyae sp. nov., Ptilocaulis ramosus sp. nov., Halichondria (Halichondria) madagascarensis sp. nov., Aaptos incrustans sp. nov., Lissodendoryx (Lissodendoryx) ingolei sp. nov., Fibulia punicea sp. nov., Clathria (Clathria) flo sp. nov. and Callyspongia (Callyspongia) cacumen sp. nov. Additionally, a range extension of Ancorina corticata Lévi, 1964, is reported. The sponge assemblage had high levels of endemism with several species shared by both the western and eastern flanks of the seamount. In contrast, sponge assemblages differed significantly according to depth, with the mesophotic zone (31–150 m) acting as a transition between the shallow (15–30 m) and submesophotic (> 150 m) zones. Species richness and the number of new species were highest in the submesophotic zone.

Modulation of the internal wave regime over a tropical seamount ecosystem by basin-scale oceanographic processes

Shallow seamounts are becoming increasingly recognised as key habitats for conservation due to their role as biological refuges, particularly throughout oligotrophic oceans. Traditionally, Taylor caps have been invoked as the mechanism driving biomass aggregation over seamounts but emerging evidence based on higher resolution measurements highlights the importance of internal waves (IW) to the local ecosystem. These waves can flush the benthic habitat with cool water from depth and impact on nutrient supply over short time scales through turbulent mixing that may also influence fish behaviour. They are dependent on the regional stratification, however, and thus influenced by planetary-scale variability in oceanographic conditions. We present here detailed observations of the internal wave regime over a shallow seamount, called Sandes, in the central Indian Ocean throughout different phases of the Indian Ocean Dipole (IOD) that modulated the regional stratification. A deep thermocline, caused by the 2019 IOD event precluded internal wave activity over the summit, whereas a thermocline collocated with the summit during 2020 when the IOD reversed polarity resulted in a 30 m amplitude internal tide signal (t ∼ 12.5 h). A shallow thermocline, observed during 2022, resulted in propagation of IWs over the summit with less visible internal tide. Harmonic analysis shows the presence of high frequency waves (t ∼ 15 min) on both flanks of the seamount during 2020 & 2022, which are likely a result of local shear instability, whereas 2019 shows an asymmetric response, potentially due to the strong background current and suppression of the thermocline beneath the depth of the summit. The potential importance of the waves over the summit to the local ecosystem may be attributed to the elevated turbulence measured at the thermocline during internal wave propagation, with ε > 10-5 W kg-1 routinely observed. Our results highlight the ability of thermocline depth to act as a gating condition for internal wave evolution over the summit. These results show that, whilst the water column exhibits variability at short spatiotemporal scales compared to the frequently cited Taylor cap dynamics, it is also regulated by the wider basin scale processes. Thus, a more integrated approach is needed when assessing these dynamic and environmentally critical habitats to include the effects of physical oceanographic controls across multiple spatiotemporal scales.

Enhanced nutrient supply promotes mutualistic interactions between cyanobacteria and bacteria in oligotrophic ocean.

Cyanobacteria can form complex interactions with heterotrophic microorganisms, but this relationship is susceptible to nutrient concentrations. Disentangling the cyanobacteria-bacteria interactions in relation to nutrient supply is essential to understanding their roles in geochemical cycles under global change. We hypothesize that enhanced nutrient supply in oligotrophic oceans can promote interactions among cyanobacteria and bacteria. Therefore, we investigated the planktonic bacteria and their interactions with cyanobacteria in relation to elevated nutrients caused by enhanced upwelling around a shallow and a deep seamount in the tropical western Pacific Ocean. We found obviously higher complexity of network occurred with significantly more cyanobacteria in the deep chlorophyll maximum layer ofthe shallow seamount when compared with that ofthe deep seamount. Cyanobacteria can shape bacterial interaction and community evenness in response to relatively high nutrient concentrations. The effects of the nutrients on cyanobacteria-related networks were further estimated based on the Tara Oceans data. Statistical analyses further showed a facilitative effect of nitrate concentrations on cyanobacteria-bacteria mutualistic interactions in the global oligotrophic ocean. By analysing the Tara Ocean macrogenomic data, we detected functional genes related to cyanobacteria-bacteria interactions in all samples, indicating the existence of a mutualistic relationship. Our results reveal cyanobacteria-bacteria interaction in response to nutrient elevation in oligotrophic ocean and highlight the potentially negative effects of global change on the bacterial community from the view of the bio-interaction.

Seamounts generate efficient active transport loops to nourish the twilight ecosystem.

Seamounts are ecological oases nurturing abundant fisheries resources and epibenthic megafauna in the vast oligotrophic ocean. Despite their significance, the formation mechanisms underlying these seamount ecological oases remain uncertain. To shed light on this phenomenon, this study conducted interdisciplinary in situ observations focusing on a shallow seamount in the oligotrophic ocean. The findings show that the seamount's topography interferes with the oceanic current to generate lee waves, effectively enhancing the nutrient supply to the euphotic layer downstream of the seamount. This continuous supply enhances phytoplankton biomass and subsequently the grazing and diurnal vertical migration of zooplankton, rapidly transporting the augmented phytoplankton biomass to the aphotic layer. Unlike the cyclonic eddies that move in the upper ocean, seamounts stand at fixed locations creating a more efficient and steady active transport loop. This active transport loop connects the euphotic and twilight zones, potentially conveying nourishment to benthic ecosystems to create stereoscopic oases in the oligotrophic ocean.

Comparing Seamounts and Coral Reefs with eDNA and BRUVS Reveals Oases and Refuges on Shallow Seamounts.

Seamounts are the least known ocean biome. Considered biodiversity hotspots, biomass oases, and refuges for megafauna, large gaps exist in their real diversity relative to other ecosystems like coral reefs. Using environmental DNA metabarcoding (eDNA) and baited video (BRUVS), we compared fish assemblages across five environments of different depths: coral reefs (15 m), shallow seamounts (50 m), continental slopes (150 m), intermediate seamounts (250 m), and deep seamounts (500 m). We modeled assemblages using 12 environmental variables and found depth to be the main driver of fish diversity and biomass, although other variables like human accessibility were important. Boosted Regression Trees (BRT) revealed a strong negative effect of depth on species richness, segregating coral reefs from deep-sea environments. Surprisingly, BRT showed a hump-shaped effect of depth on fish biomass, with significantly lower biomass on coral reefs than in shallowest deep-sea environments. Biomass of large predators like sharks was three times higher on shallow seamounts (50 m) than on coral reefs. The five studied environments showed quite distinct assemblages. However, species shared between coral reefs and deeper-sea environments were dominated by highly mobile large predators. Our results suggest that seamounts are no diversity hotspots for fish. However, we show that shallower seamounts form biomass oases and refuges for threatened megafauna, suggesting that priority should be given to their protection.

A Deep Seamount Effect Enhanced the Vertical Connectivity of the Planktonic Community Across 1,000 m Above Summit

AbstractSeamount effects, which are generally defined as hydrographic disturbances caused by topography and nutrient enrichment and biological aggregations around seamounts, are normally observed in shallow seamounts due to limited sampling efforts in deep seamounts. However, it remains unclear how and to what extent do deep seamounts leave their imprint on planktonic communities. Herein ciliates, a representative protist group, were chosen to explore the effect of deep seamount on planktonic community. By investigating the vertical and horizontal distribution of ciliate communities around the Kocebu Guyot (summit at −1,198 m) and in nonseamount area, we revealed an obvious deep seamount effect, which enhanced the vertical mixing of ciliate communities to an extent of over 1,000 m above the summit. The vertical mixing was manifested by a strong uplift of bottom dwellers from waters deeper than 500 m and a weak uplift from the 300 m layer to the deep chlorophyll maximum (about 150 m) layer. Network analysis showed that the ciliate cooccurrence relationship around the seamount was much more complex than that in nonseamount area. Statistical analysis indicated that seamount significantly weakened the limitation that water depth posed on vertical ciliate distribution. Overall, the ciliate communities presented a much higher‐resolution record of deep seamount effects than physico‐chemical data. Deep seamount could enhance the vertical mixing of waters and cooccurrence complexity of planktonic community to the euphotic layer. Considering the wide existence of deep seamounts, such an effect may have ecological significance and enhance the cycles of matter and energy of global oceans.

Diversity and Distribution of Harmful Algal Bloom Species from Seamount to Coastal Waters in the South China Sea.

Mount Xianbei is one of the largest shallow seamounts located in the middle of the South China Sea (SCS), which might play a role in shaping the biodiversity of surrounding continental coastal waters, particularly the diversity of phytoplankton species causing frequent harmful algal blooms (HABs) in northern SCS. However, the diversity, composition, and distribution of phytoplankton species in the seamount regions of Xianbei remain largely unexplored. In this study, samples around and outside the seamount regions were collected during a late summer cruise of 2021 to test whether seamounts play a role in HAB species propagation. In total, we identified 19 HAB species across all samples using the ASV-based DNA metabarcoding approach, 6 of which had not been reported previously in the SCS, suggesting a diverse HAB species in the SCS. Specifically, 16 HAB species were found in the seamount region of Xianbei, and 5 of them were also found in the coastal waters, indicating a close connection between seamount and coastal waters. This study was the first attempt to explore HAB species' spatial diversity and vertical distribution in the seamount region of Xianbei at single-nucleotide resolution, which provides a novel explanation for the coastal HAB occurrence in the northern SCS. IMPORTANCE There are a number of seamounts under the water of the South China Sea (SCS). The seamounts might play a role in shaping the biodiversity of surrounding continental coastal waters. However, there is no direct evidence revealing the relationship of the biodiversity of phytoplankton between seamounts and coastal waters in the SCS, especially those species having the potential to form harmful algal blooms (HABs). Some HAB species might proliferate in certain geographic locations, while others may be broadly distributed across oceanic provinces. In this study, we provided a detailed analysis of phytoplankton composition and molecular detection of HAB species from seamount to coastal waters in the SCS, which suggested a strong interaction in the HAB species between the two areas. This finding provides new insights into the diversity and distribution of HABs in seamounts and their role in shaping the composition and the occurrence of HABs in coastal water.

Relationship and stratification of multiple marine ecological indicators: A case study in the M2 seamount area of the Western Pacific Ocean

Seawater stratification is an important ecological indicator and typical feature in the marine environment, which is of great significance to the material cycle and distribution of organisms. In March 2016, a survey was conducted in the M2 seamount of the Western Pacific Ocean, seawater stratification of temperature, salinity, dissolved oxygen (DO), nutrients, bacteria, and the impact of the M2 seamount were discussed. It is found that the seawater stratification of each parameter is obvious in this area. At the depth of 0–75 m, there is a mixed layer, and these parameters remain relatively stable; at 75–500 m, there are obvious thermocline, gradient layer of bacteria, and nutricline; at the depth deeper than 500 m, these parameters remain relatively stable. In addition, at 75–150 m, there is a high-salt zone dominated by the North Pacific Tropic Water, while at 270–1370 m, there is an oxygen minimum zone with low DO. There is a close relationship in seawater stratification of different parameters. At 75–500 m, the density gradient dominated by the thermocline limits the water exchange, then directly controls the gradient layer of DO, nutrients, and bacteria. Additionally, the double actions of the high-salt zone and thermocline affect the stratification of other parameters. At 0–75 m, due to the abundant DO, sufficient carbon source, and low nutrients, the bacteria could grow largely, while at 75–500 m, bacteria could consume a large amount of DO to decompose organic matter, leading to a decrease in DO and an increase of nutrients. Although the M2 seamount is a shallow seamount, there is no upwelling around it, which weakly influences the seawater stratification of different parameters. This study will provide a new perspective on seawater stratification and its ecological effects.

Simulations of long-crested wind waves over the shallow seamount at Glagah

In this paper, we investigate long-crested wave evolution over a cross-section of non-monotone bathymetry. The bathymetry is from an area with an isolated shallow seamount on the south coast of Java Indonesia near Glagah port. We will investigate waves with a significant wave height (Hs) of 2 m and more energetic waves of 4 m over a cross-section through the top of the seamount that is only 2 m under sea level and located 8 km from the coast. The wave simulations over a 12 km long cross-section to the beach are used to determine the numerical and physical aspects of the wave transformation over bathymetry, including breaking properties, the effect of bottom friction, and differences between second and third order simulations. This study used HAWASSI software for irrotational fluid dynamics with a Fourier spectral method implementation. Crossing the top of the seamount leads to a transformation from the wind waves into a downward running flow described by very long infragravity (IG) waves that persist until the nearshore area with the run-up area of the coast.

Biogeochemical characteristics and microbial response to indicate degradation of organic matter around Pair-summit Seamounts in the Tropical Western Pacific Ocean

The ecological knowledge of seamounts has accumulated plentifully in recent years. However, there are few studies on the distribution and degradation pattern of organic matter in seamounts morphology by using microorganisms and nutrients as indicators. In this research, we investigated the dynamic distribution of nutrients, particulates and microbial communities of the summits, flanks and base in the M5 Pair-summit Seamount, aiming to better understand the distribution and degradation pattern of organic matter around the seamount. The results indicate that TOC concentration presented a gradual decrease from summit to base, and obtained the lowest value at Base1 (0.65 mg L−1). Illumina Hiseq high-throughput sequencing analysis shows that Firmicutes (38.86%) and Bacteroidetes (6.86%) on the seamount Base1 obtained the highest relative abundance, which were related to the degradation of organic matter caused by denitrification. The distribution of organic matter and composition of microbial communities formed disparity due to the morphology of seamounts. In addition, shallow seamounts and deep seamounts exhibited a high degree of temporal and spatial specificity. Summit1 produced organic matter from phytoplankton, with the highest TOC content (1.23 mg L−1), dominated in microorganisms of Cyanobacteria (18.06%), and an upwelling of nutrients due to the “seamount effect”. However, Summit2 utilized nitrifying bacteria Taumarchaeota to degrade organic matter. This research suggests that the unique topographic features of seamounts is essential to sustain the surrounding microbial composition and hydrodynamics, which can further promote the study of seamount ecological environment.

Distribution and biomass of gelatinous zooplankton in relation to an oxygen minimum zone and a shallow seamount in the Eastern Tropical North Atlantic Ocean

Physical and topographic characteristics can structure pelagic habitats and affect the plankton community composition. For example, oxygen minimum zones (OMZs) are expected to lead to a habitat compression for species with a high oxygen demand, while upwelling of nutrient-rich deep water at seamounts can locally increase productivity, especially in oligotrophic oceanic waters. Here we investigate the response of the gelatinous zooplankton (GZ) assemblage and biomass to differing oxygen conditions and to a seamount in the Eastern Tropical North Atlantic (ETNA) around the Cape Verde archipelago. A total of 16 GZ taxa (>1100 specimens) were found in the upper 1000 m with distinct species-specific differences, such as the absence of deep-living species Atolla wyvillei and Periphylla periphylla above the shallow seamount summit. Statistical analyses considering the most prominent groups, present at all stations, namely Beroe spp., hydromedusae (including Zygocanna vagans, Halicreas minimum, Colobonema sericeum, Solmissus spp.) and total GZ, showed a strong positive correlation of abundance with temperature for all groups, whereas oxygen had a weak negative correlation only with abundances of Beroe spp. and hydromedusae. To account for size differences between species, we established length-weight regressions and investigated total GZ biomass changes in relation to physical (OMZ) and topographic characteristics. The highest GZ biomass was observed at depths of lowest oxygen concentrations and deepest depth strata at the southeastern flank of the seamount and at two stations south of the Cape Verde archipelago. Our data suggest that, irrespective of their patchy distribution, GZ organisms are ubiquitous food web members of the ETNA, and their habitat includes waters of low oxygen content.

A first assessment of the distribution and abundance of large pelagic species at Cocos Ridge seamounts (Eastern Tropical Pacific) using drifting pelagic baited remote cameras.

Understanding the link between seamounts and large pelagic species (LPS) may provide important insights for the conservation of these species in open water ecosystems. The seamounts along the Cocos Ridge in the Eastern Tropical Pacific (ETP) ocean are thought to be ecologically important aggregation sites for LPS when moving between Cocos Island (Costa Rica) and Galapagos Islands (Ecuador). However, to date, research efforts to quantify the abundance and distribution patterns of LPS beyond the borders of these two oceanic Marine Protected Areas (MPAs) have been limited. This study used drifting-pelagic baited remote underwater video stations (BRUVS) to investigate the distribution and relative abundance of LPS at Cocos Ridge seamounts. Our drifting-pelagic BRUVS recorded a total of 21 species including elasmobranchs, small and large teleosts, dolphins and one sea turtle; of which four species are currently threatened. Depth of seamount summit was the most significant driver for LPS richness and abundance which were significantly higher at shallow seamounts (< 400 m) compared to deeper ones (> 400m). Distance to nearest MPA was also a significant predictor for LPS abundance, which increased at increasing distances from the nearest MPA. Our results suggest that the Cocos Ridge seamounts, specifically Paramount and West Cocos which had the highest LPS richness and abundance, are important aggregation sites for LPS in the ETP. However, further research is still needed to demonstrate a positive association between LPS and Cocos Ridge seamounts. Our findings showed that drifting pelagic BRUVS are an effective tool to survey LPS in fully pelagic ecosystems of the ETP. This study represents the first step towards the standardization of this technique throughout the region.

Erratum to: Analysis of differences in nutrients chemistry in seamount seawaters in the Kocebu and M5 seamounts in Western Pacific Ocean

Comprehensive surveys were conducted in the Kocebu deep seamount and the M4 shallow seamount in the Western Pacific Ocean in March 2018 and May 2019, respectively. The distributions of nutrients in euphotic zone of the two seamount-areas were revealed, and the causative controlling factors were analyzed. Results show that the vertical distribution of nutrients in the two seamount-areas accorded with the general law of the oligotrophic ocean. The concentrations of NO3-N, PO4-P, and SiO3-Si generally increased gradually with the increase of water depth, and they were extremely low in water layers within 100 m. The area of high NO2-N concentration well agreed with the Deep Chlorophyll Maximum Layer. On the other hand, the distribution of water masses and phytoplankton and hydrological conditions in the two seamount-areas were different, resulting in lower concentrations of NO3-N, PO4-P, and SiO3-Si in the water layers below 100 m in the Kocebu seamount area than those in the M4 seamount area. In addition, NO2-N was affected by the distribution of phytoplankton, and distributed mainly in the water layers of 150 and 100 m. There was upwelling in the euphotic zone of M4 seamount area, causing accumulations of nutrients and phytoplankton around the seamount, forming a “seamount effect”; however, no such an effect was found in Kocebu seamount area. Affected by the composition of biological community and the “seamount effect”, the nitrogen limitation in the M4 seamount area was not significant, and the dissolved inorganic nitrogen (DIN):PO4-P and SiO3-Si:DIN were closer to the Redfield ratios.

Spatial Variation in Pelagic Wildlife Assemblages in the Ascension Island Marine Protected Area: Implications for Monitoring and Management

Bathymetric features such as islands and seamounts, as well as dynamic ocean features such as fronts often harbour rich marine communities. We deployed mid-water baited remote underwater video systems on three expeditions in Ascension Island’s Exclusive Economic Zone (EEZ), surveying the waters associated with six different bathymetric and dynamic ocean features: Ascension Island, two shallow seamounts (summits ≤ 101 m), one deeper seamount (summit &amp;gt; 250 m), apparent fronts, and haphazardly sampled open ocean areas. At Ascension Island, the pelagic assemblage consisted of a moderate proportion of predators and a diverse range of other taxa, including turtles, dolphins, and large non-piscivores. At the two shallow seamounts, sharks, tunas, billfish, and other large pelagic predators formed the vast majority of the assemblage, contributing &amp;gt; 99.9% of biomass and &amp;gt; 86% of abundance. At the deeper seamount, the pelagic community was comparatively depauperate, however the functional composition of its assemblage indicated some similarities to the shallow seamounts. Apparent fronts did not significantly differ from random offshore sites for metrics such as total abundance and taxonomic richness. However, they harboured assemblages with more abundant sharks, tunas, and large piscivores than random ocean open locations and these differences may be driven by certain front-associated species. Our results illustrate that pelagic assemblages vary markedly among different physical and oceanographic features and that seamounts appear particularly important for pelagic predators. The diversity and abundance of the assemblage, as well as the threatened status of many of the species observed, serve to highlight the conservation value of the Ascension Island EEZ. Our results also provide important baseline information of pelagic wildlife assemblages against which the performance of the recently implemented Ascension Island Marine Protected Area can be evaluated.

Dynamics of currents and biological scattering layers around Senghor Seamount, a shallow seamount inside a tropical Northeast Atlantic eddy corridor

Biophysical interactions of the mesopelagic fauna and hydrodynamic features were investigated at a shallow, tall seamount in the tropical Northeast Atlantic based on data from three inter-disciplinary surveys collected between 2009 and 2015. Senghor Seamount is located close to the equator in a unique but underexplored oceanographic setting, where classical seamount effects are expected to be small. Shipboard Acoustic Doppler Current Profiler (ADCP) and remote sensing data revealed flow phenomena strongly varying over different temporal and spatial scales. The quasi-steady, longer period flow at the seamount was largely unchanged by flow-topography interaction with the exception of a persistent and recurring shadow zone of weaker currents downstream of the summit. Senghor Seamount is exposed to energetic mesoscale eddy activity in surrounding waters with potentially large physical and biological implications. At higher frequencies, tidal currents interacting with the seamount generated large-amplitude internal waves propagating horizontally and vertically away from the summit. Analysis of acoustic backscatter data and micronekton biomass displayed prominent near-surface night time and deep (>400 m) day time scattering layers associated with the diurnal vertical migration of the mesopelagic fauna. An intense and previously unreported aggregation of acoustic scatterers was detected in the depth range 150–300 m inside a radius of 3 km along the upper slopes of the seamount summit. ADCP measurements over a full semi-diurnal tidal cycle suggest that biophysical interactions were present in near-seafloor waters around the upper seamount as evidenced by plankton distributions that were modified by topographic blocking during descent and aggregation in waters characterised by internal waves and enhanced tidally-driven energy dissipation.

Shallow seamounts represent speciation islands for circumglobal yellowtail Seriola lalandi

Phenotypic plasticity in life-history traits in response to heterogeneous environments has been observed in a number of fishes. Conversely, genetic structure has recently been detected in even the most wide ranging pelagic teleost fish and shark species with massive dispersal potential, putting into question previous expectations of panmixia. Shallow oceanic seamounts are known aggregation sites for pelagic species, but their role in genetic structuring of widely distributed species remains poorly understood. The yellowtail kingfish (Seriola lalandi), a commercially valuable, circumglobal, epipelagic fish species occurs in two genetically distinct Southern Hemisphere populations (South Pacific and southern Africa) with low levels of gene-flow between the regions. Two shallow oceanic seamounts exist in the ocean basins around southern Africa; Vema and Walters Shoal in the Atlantic and Indian oceans, respectively. We analysed rare samples from these remote locations and from the South African continental shelf to assess genetic structure and population connectivity in S. lalandi and investigated life-history traits by comparing diet, age, growth and maturation among the three sites. The results suggest that yellowtail from South Africa and the two seamounts are genetically and phenotypically distinct. Rather than mere feeding oases, we postulate that these seamounts represent islands of breeding populations with site-specific adaptations.

The Fate of Deep-Sea Coral Reefs on Seamounts in a Fishery-Seascape: What Are the Impacts, What Remains, and What Is Protected?

Environmental harm to deep-sea coral reefs on seamounts is widely attributed to bottom trawl fishing. Yet, accurate diagnoses of impacts truly caused by trawling are surprisingly rare. Similarly, comprehensive regional assessments of fishing damage rarely exist, impeding evaluations of, and improvements to, conservation measures. Here we report on trawling impacts to deep-sea scleractinian coral reefs in a regional (10–100s of km) fishery seascape off Tasmania (Australia). Our study was based on 148 km of towed camera transects (95 transects on 51 different seamounts with 284,660 separate video annotations and 4,674 on-seamount images analysed), and commercial trawling logbook data indexing fishing effort on and around seamounts. We detect trawling damage on 88% (45 of 51) of seamounts. Conversely, intact deep-sea coral reefs persist in refuge areas on about 39% (20 of 51) of the seamounts, and extend onto rocky seabed adjacent to seamounts. Depth significantly shapes the severity of trawl damage. The most profound impacts are evident on shallow seamounts (those peaking in 1,500 m) are beyond the typical operating depth of the trawl fishery and exceed the depth range of living deep-sea coral reefs in the region. Accurately diagnosing the nature and extent of direct trawling impacts on seamount scleractinian coral reefs must use stringent criteria to guard against false positive identifications of trawl impact stemming from either (1) misidentifying areas that naturally lacked deep-sea coral reef as areas where coral had been removed, or (2) attributing trawling as the cause of natural processes of reef degradation. The existence of sizeable deep-sea coral reef refuges in a complex mosaic of spatially variable fishing effort suggests that more nuanced approaches to conservation may be warranted than simply protecting untrawled areas, especially when the biological resources with conservation value are rare in a broader seascape context.

Biodiversity of Isopoda and Cumacea (Peracarida, Crustacea) from the Marine Protected Area Namuncurá-Burdwood Bank, South-West Atlantic

Burdwood Bank is a shallow seamount located south of the Malvinas (Falkland) Islands and east of Tierra del Fuego. In 2013, the area down to 200 m depth of this bank was declared the first open-sea (non-coastal) Marine Protected Area (MPA) in Argentina, and named “Namuncurá” (MPAN-BB). The present study aimed to evaluate the biodiversity of the isopods and cumaceans from the MPAN-BB. The studied material was collected on board the Argentine RV Puerto Deseado during the Campaña Antártica de Verano 2013. Three stations from the MPAN-BB, respectively, located in the core, buffer, and transition sectors, were sampled. A total of 17,076 specimens were sorted and 70 taxa distributed in 22 families were identified. Thirty-five new records of distribution and eight species most probably new to science are herein reported. Paramunnidae and Diastylidae were the most abundant and diverse families among isopods and cumaceans, respectively. Our records, together with those obtained from the literature, make a total of 55 nominal species so far known from Burdwood Bank. Except for one nominal species, which had been reported from the Puerto Rico Trench and the Argentine Basin, all the remaining species had also been recorded from other localities within the Magellan Region. In contrast, only nine out of the 55 nominal species had also been reported from the Antarctic Peninsula and/or Scotia Arc. This indicates that the isopods and cumaceans from Burdwood Bank are typically Magellanic, and there is little correspondence between this fauna and that from the Antarctic Peninsula and Scotia Arc.

The seamount effect on phytoplankton in the tropical western Pacific

The tropical western Pacific, one of the most oligotrophic seas on Earth, has a large number of seamounts. In this sea, we hypothesize that although the presence of shallow seamounts can enhance phytoplankton biomass, they do not alter the phytoplankton community composition. The concentration and distribution of size-fractionated chlorophyll a (Chl a) on the C4 Seamount were measured, and the phytoplankton community was analyzed based on photosynthetic pigments. The results showed that the deep chlorophyll maximum (DCM) was located between 100 and 150 m and that picophytoplankton such as prochlorophytes and cyanobacteria (mainly Synechococcus) were the dominant groups, contributing approximately 85% of the Chl a. The integrated Chl a concentration above the seamount was significantly higher than that off the seamount, and most communities were enhanced near the peak. The physical, chemical, and biological coupling processes of the C4 Seamount support the classic hypothesis of the seamount effect. However, the relative contributions of different phytoplankton groups to Chl a were not significantly different between stations above and off the seamount, and the analysis of similarities among results also indicated that the community composition on the C4 Seamount did not differ significantly from that off the seamount. In addition, size-fractionated Chl a data from two nearby shallow seamounts also supported our hypothesis. More studies on the ecology of seamounts in the tropical western Pacific are required to further confirm and refine these results.

Evidence for long-term seamount-induced chlorophyll enhancements

Seamounts are ubiquitous global features often characterized by biological hotspots of diversity, biomass, and abundance, though the mechanisms responsible are poorly understood. One controversial explanation suggests seamount-induced chlorophyll enhancements (SICE) subsidize seamount ecosystems. Using a decade of satellite chlorophyll data, we report substantial long-term chlorophyll enhancements around 17% of Pacific seamounts and 45% of shallow (< 100 m) seamounts, with the highest probability of detection at shallow, low-latitude seamounts. SICE is shown to enhance chlorophyll concentrations by up to 56% relative to oceanic conditions, and SICE seamounts have two-fold higher fisheries catch relative to non-enhancing seamounts. Therefore, seamount-induced bottom-up trophic subsidies are not rare, occurring most often at shallow, heavily exploited seamounts, suggesting an important subset of seamounts experience fundamentally different trophic dynamics than previously thought.